Merge branch 'http-external-deps' into 'master'

Use HTTPS for external dependencies

As noted by @NickMilef in the developers mailing list, `googletest` git repository was using the SSH protocol instead of HTTPS, which requires users to add an ssh_key on github.com.

We switch all those external projects to HTTPS protocol, but keep the ones hosted by ourselves on gitlab.kitware.com in SSH since we might be contributing back to those during development

See merge request !105